1. Field of the Invention
The present invention relates to a method for forming a SOI-type substrate. More particularly, the present invention relates to a method for forming a strained silicon layer on a Silicon-germanium On Insulator substrate (hereinafter referred to as “a SOI substrate”).
2. Description of the Related Art
A SOI substrate is obtained by forming a buried oxide layer in a substrate to form a device isolation layer to completely isolate devices from each other. Due to its tolerance to radiation and ability to elevate a breakdown voltage, SOI substrates are mainly used to fabricate semiconductor devices such as high-performance processors.
In case that the SOI substrate includes a SOI layer composed of silicon-germanium and a silicon layer formed on the SOI layer, a difference in lattice constant between the silicon layer and the silicon-germanium layer renders a biaxial tension to the silicon layer. If the silicon layer is strained, electron carriers decrease in effective mass and scattering, and this causes an increase in mobility of the electron carriers in the silicon layer.
The prior art discloses conventional methods for improving performance of a circuit of a CMOS semiconductor device. The methods include forming a SOI layer composed of silicon-germanium, then forming a strained silicon layer on the SOI layer. Referring to FIG. 1, the conventional method includes stacking a graded silicon-germanium (SixGe1−x) layer 13, a relaxed silicon-germanium (Si1−yGey) buffer layer 14, a phosphorus doped silicon-germanium (Si1−yGey) layer 15, a strained silicon (Si) channel layer 16, a strained silicon-germanium (Si1−zGez) channel layer 17, a relaxed silicon-germanium (SiyGe1−y) layer 18, and a silicon layer 19 on a first silicon substrate 12 using an epitaxial growth method.
Thereafter, a second silicon substrate 26 where an oxide layer 28 is formed is bonded to the first silicon substrate 12 where the silicon layer 19 is formed, such that top surfaces of the first and second silicon layers 12 and 26 are in contact with each other. Reference numeral 20 in FIG. 1 represents a contact surface between the first and second silicon substrates 12 and 26.
The first silicon substrate 12, the graded silicon-germanium layer 13, and the relaxed silicon-germanium buffer layer 14, are removed from the first silicon substrate 12 by using a high-temperature solution of potassium hydroxide. The phosphorus doped silicon-germanium layer 15 is oxidized by wet oxidation and then removed using fluoric acid.
As a result, the strained silicon channel layer 16 remains on top of the resultant structure including the two substrates bonded to each other. In this method, when the strained silicon channel layer 16 has a thickness of approximately 40 to 100 angstroms, both sides of the strained silicon channel layer 16 are in contact with the silicon-germanium layers 15 and 17, which are different from each other in lattice constant. Thus, lattice defects caused by interface misfit may be generated in the strained silicon channel layer 16. Furthermore, removing the phosphorus doped silicon-germanium layer 15 may adversely influence the quality of the strained silicon channel layer 16, where devices will be formed in subsequent processes.
Generally, if layers having different lattice constants are stacked to more than a certain thickness, pressure or tension applied to the layers is relaxed by generation of the lattice defects. However, if a silicon layer is stacked on the relaxed silicon-germanium layer using an epitaxial growth method, the quality of the silicon layer deteriorates due to lattice defects in the silicon-germanium layer. Consequently, semiconductor devices using the epitaxial silicon layer may suffer from poor operational characteristics.
In the event that a strained silicon layer is formed on a SOI substrate using a silicon-germanium layer, an increase in electron mobility affected by tension ceases when the germanium concentration is 10% or more, whereas hole mobility affected by tension continues to increase until the germanium concentration reaches 30%. Accordingly, to improve performance of devices, it is necessary to increase the germanium concentration of the silicon-germanium layer. Meanwhile, when a buried oxide layer is formed using separation by implanted oxygen (SIMOX), a high-temperature annealing should follow. However, as a germanium concentration increases, a melting point of a substrate decreases. Therefore, it undesirable to employ SIMOX when forming an SOI substrate using a silicon-germanium layer having a strained silicon layer.
A method as illustrated in FIGS. 2 to 5 may be used to form a SOI substrate that does not suffer from interface misfit. The SOI substrate includes a SOI layer of high germanium concentration having a strained silicon layer on its surface. According to the method, as illustrated in FIG. 2, a relaxed thick silicon-germanium layer 34 is formed on a first silicon substrate 32. Hydrogen ions are then doped into the first silicon substrate 32 including the relaxed thick silicon-germanium layer 34. The resultant structure is then annealed. Thus, lattice defects, which result from misfit interface 38 between the first silicon substrate 32 and the silicon-germanium layer 34, are cured around a surface 36 where a hydrogen concentration is high.
As illustrated in FIG. 3, after forming an oxide layer 48 on a second silicon substrate 46, the first and second silicon substrates 32 and 46 are bonded to each other. Reference numeral 50 in FIG. 3 represents a contact surface between the substrates 32 and 46.
As illustrated in FIG. 4, the whole bonded substrate is cut at the surface 36 where the hydrogen concentration is high. As a result, the second silicon substrate 46 including a silicon oxide layer 48 and the relaxed silicon-germanium layer 34 is separated from the first silicon substrate 32. In this case, lattice defects in the silicon-germanium layer 34 may be partially cured by the high hydrogen concentration at the cut surface 36.
As illustrated in FIG. 5, a silicon layer is thinly formed on the cut surface using an epitaxial growth method. Thus, a strained silicon layer 52 is formed on the silicon-germanium layer 34
However, in this method, the silicon-germanium layer 34 disposed under the strained silicon layer 52 may suffer from lattice defects during the hydrogen ion implantation process. As a result, hydrogen may not properly cure lattice defects of the relaxed silicon-germanium layer.